Natural Hazards: GEOG3

There are 3 topic areas you need to cover for the GEOG3 exam, on the AQA syllabus, the ones we have decided to study in detail are:

· World Cities

· Weather and Climate

· Natural Hazards

Natural Hazards: A Course Overview

The main headings for this topic are:

1.      Plate Movement

2.      Vulcanicity

3.      Seismicity

The following are the case studies that we will talk about and compare synoptically:

Vulcanicity

Montserrat, Caribbena (LEDC)

Mount Etna, Sicily (MEDC)

Seismicity

Haiti Earthquake (LEDC)

L'Aquila Earthquake (MEDC)

Both these earthquakes can be synoptically linked to North Ridge and Christchurch

Boxing Day Tsunami 2004, Indonesia

Japan 2011 Tsunami

Both tsunamis can be synoptically linked to Christchurch

Plate Movement

· Plate Tectonic Theory

· Features of Plate Margins

· Hot Spots

Vulcanicity

· Distribution

· Volcanic Eruptions

· Intrusive + Extrusive Volcanic Activity in the UK

· Impacts of Volcanic Activity

· Case Studies

Seismicity

· Causes of earthquakes

· Distribution

· Magnitude + Frequency

· Effects of Earthquakes

· Case Studies

Key Terms

Hot Spot: A point on the surface of the Earth located above a plume of rising magma.

Plate: The lithosphere is divided into a number of segments known as plates. These rigid slabs float on the underlying semi-molten mantle and are moved by convection currents within in.

Plate Tectonics: A theory that attempts to explain the formation and distribution of the Earth's major structural features in terms of a series of plates that make up its surface.

Sesimic Waves: Shock waves released by the rupture of rock strats at the focus of an earthquake. They travel through the rocks and are measured and recorded on a seismograph.

Tsunami: Sea waves, which can be very large, generated by shallow-focus underwater earthquakes, volcanic eruptions.

Volcano: An opening or vent through which magma, molten rock, ash or volatiles erupt on to the surface of the Earth.

Plate Tectonic Theory

· 1912 German, Alfred Wegener published a theory stating that 300million years ago the world was just a single continent.

· This single continent was named Pangaea, but it later split into 2 continents named, Laurasia(North) and Gondwanaland (South)

· He claimed there was both geological and biological evidence to support his theory.

Geological evidence:

· Evidence of glaciation in the Carboniferous period where deposits have been found in S. America, Antartica and India = they must have been formed together but then moved

· Rock sequences in N. Scotland which agree with some found in E. Canada (meaning they came from the same origin)

Biological evidence:

· Fossil brachiopods found in Indian limestone compared with similar fossiles found in Australia

· Fossil remains of reptiles found in both S. America and S. Africa

· Fossil remains of plants which existed when coal was being formed have only been located in India and Antartica

This evidence does NOT explain any continental movement but from 1940 evidence mounted up which show Wegener could have been right! This is due to the Mid-Atlantic Ridge being found, why does this matter?

· It suggested the Ocean floor was spreading (evidence for this = alternating polarity of the rocks forming the ocean crust which give magnetic 'stripes' in the rocks also the crust gets older with distance from the ridge)

· Sea floor spreading implies the Earth is getting bigger. But us all being bright sparks know this is nonsense therefore somewhere plates must be gettind destroyed to accomodate the increasing size! - evidence = Oceanice Trenches

Above is The Earth's Layers. http://www.youtube.com/watch?v=3MFr2cC3erk&feature=related This link leads you to an animated video which describes each layer of the Earth!

Plate Margins

· Constructive (diverging)

· Destructive (converging)

· Conservative

CONSTRUCTIVE MARGINS:

· Plates moving AWAY from each other due to convection currents rising and diverging

· Produces: RIFT VALLEYS in continental crust and RIDGES in oceans

Rift Valleys: E.G. The African Rift Valley, extending 4,000km and 50kn wide and 600m deep

Ocean Ridges: Can be up to 60,000km long and 50km wide. With a slow annual growth of 10-15mm a year. Volcanic activity can occur, E.G. Surtsey, Nr. Iceland created in 1963 by magma.

DESTRUCTIVE MARGINS

Destructive Plate Margins

There are 3 types of destructive plate margins:

1. Oceanic &Continental

2. Oceanic & Oceanic

3. Continental & Continental

Oceanic & Continental Plate margins

· Denser oceanic plate is forced under the lighter continental plate by subduction.

· Forms a deep oceanic trench.

· As plate descends the pressure triggers earthquakes (with deep focus’) along the line of subduction.

· Heat and friction melts the subducting melting plate into magma in the beneioff zone.

· Now less dense, it rises in plutons of magma and forms volcanoes on the surface.

· Offshore there may be a series of volcanoes, known as an island arc.

Oceanic &Oceanic convergent

· Also have ocean trenches and island arcs

Continental & Continental convergent

· Continental crust has a much lower density than underlying layers so is subducting at their meeting.

· Less subduction = less volcanic activity

· Their edges and sediment between is forced up to fold mountians

· Shallow-focus earthquake and deep mountain roots

CONSERVATIVE MARGINS

· 2 plates SLIDING PAST EACH OTHER, parallel to each other.

· Movement causes stress to build up causing shallow-focus earthquakes and transform faults to develop but NO volcanic activity occurs.

· E.G. North Ridge, San Andreas Fault, LA

Hot Spots

· Stationary

· Evidence for Wegeners theory

· Radioactive elements inside the mantle cause plume of magma to rise,if it breaks at the surface = A VOLCANO

· E.G. Hawaiian Islands

Seismicity

Seismicity is the study of earthquakes.

Focus = where pressure is released

· Different types of depths of focus

· Shallow (0-70km)

· Deep (700km)

Seismic waves radiate out at different speeds:

· Primary (P) Waves: FASTEST, travel out, vibrating in the direction on travel

· Secondary (S) Waves: 1/2 speed of P waves, Vibrate out at RIGHT ANGLES of travel

· Surface (L) Waves: Slowest, Some in a rolling motion

Magnitude + Frequency

Richter Scale

Measures MAGNITUDE

Mercalli Scale: Measures intensity of the impacts on people and structures

Seismographs record the AMPLITUDE of earthquakes using a LOGARITHMIC SCALE whereby each interval indicates a X10 increase

Factors Influencing The Impact of A Quake

1.      Type + Structure of buildings

2.      Populations density

3.      Location (in relation to plate boundary)

4.      Distance from Epicentre

5.      Magnitude

6.      Planning, Prediction + Preparation

7.      Emergency services

8.      Level of development

9.      Time of Day

10.  Type of Bed Rock

Vulcanicity

· Volcanic activity occurs along plate margins (constructive and destructive), ocean ridges, near rift valleys or on/near subduction zones, and over hot spots.

· Volcanoes vary according to their forms, frequency and type of eruption.

· These are related to the type or plate margin, emission or type of lava.

INTRUSIVE VOLCANIC LANDFORMS

·         When magma reaches the surface the majority of it is intrusive and blows the surface if the earth.

It solidifies forming a number of features – exposed over time by erosion.

·         Batholiths are formed deep below the surface when large amounts of magma cool and solidify like granite.

-          It is a dome shape

-          Several hundred km across

-          Area surrounding the batholith is altered by heat & pressure forming metamorphic rock = Aureole

-          Laccolith occurs when there is a small injection of magma which intrudes the layers of rock. The overlaying rick is then forced to arch up.

·         Dykes are vertical intrusions with horizontal cooling cracks. 

-          Dykes swarms when they are together – e.g. Isles of Arran Scotland

-          Made of more resistant material

·         Sills are horizontal intrusions along the bedding plains

-          Vertical cooling cracks

-          Made of dolerite rock

EXTRUSIVE VOLCANIC LANDFORMS

· Involves 2 types of lava:

· Basaltic: formed from magma low in silica, fluid magma, prevents sudden explosiveness

· Andesitic/Rhyolitic: formed from magma rich in silica, very viscous, violent explosive

The main types of extrusive volcanic landforms:

· Lava Plateaux - formed from fissure eruptions, lava flows are basaltic in nature so flow for miles

· Basic/Shield Volcano - formed from free-flowing lava. Gentle sides, cover large surface area

· Acid/Dome Volcano - steep-sided convex cones, viscous lava

· Ash and Cinder Cones - formed from ash, cinders & volcanic bombs ejected from crater, steep and symmetrical sides

· Composite Cones - pyramid shape, layers of ash and lava

· Calderas - occur when gas builds up and huge explosion removes cone summit = a hole, may become flooded by the sea from within it

Minor Volcanic Forms



· Solfatara - small volcanic areas without cones, produced by gases escaping the surface

· Geysers - occur when water , heated explodes onto the surface

· Hot springs/Boiling mud - sometimes water heated below does not explode

· Fumaroles - super heated water, turning to steam as pressure drops when it emerges from the ground

 

 OUR NEXT BLOG WILL BE AN OVERVIEW OF THE CASE STUDIES FOR THIS SECTION...ENJOY GEOGRAPHY!!! (: